1. Field of the Invention
The present invention relates generally to micro-optical systems realized by micromachining technology, and more particularly to a deflector mirror of a type that causes a mirror substrate to vibrate in a reciprocating manner around torsion beams as a torsional rotary shaft, the torsion beams each having one end thereof supported. The present invention also relates to apparatuses such as optical scanning devices and image forming apparatuses to which the deflector mirror is applied. The present invention relates to a technique suitable for the writing unit of an image forming apparatus such as an electrophotographic printer or copier.
2. Description of the Related Art
A deflector mirror in which a mirror substrate supported by two beams provided on a straight line is caused to vibrate in a reciprocating manner around the two beams serving as a torsional rotary shaft by the electrostatic attraction between the mirror substrate and electrodes positioned opposite the mirror substrate is disclosed in Petersen, K. E.; “Silicon Torsional Scanning Mirror,” IBM Journal of Research and Development, 24, 631-637 (1980). Compared with the conventional optical scanning device that causes a polygon mirror to rotate using a motor, this deflector mirror formed by micromachining technology has a simple structure, and can be formed by batch formation in a semiconductor process. As a result, this deflector mirror is easy to reduce in size and manufacturing cost. Further, this deflector mirror uses a single reflecting surface. Accordingly, unlike the polygon mirror, there are no variations in accuracy between reflecting surfaces. Further, since the deflector mirror performs scanning in a reciprocating manner, it is possible to support an increase in speed.
Such electrostatically driven torsional vibration deflector mirrors are disclosed as follows. Japanese Patent Nos. 2924200 and 2981600 each disclose a deflector mirror (an optical scanner) that has beams each shaped like an S letter to reduce rigidity so that a great deflection angle can be obtained with a little driving force. Japanese Laid-Open Patent Application No. 7-92409 discloses an optical scanner in which beams are thinner than a mirror substrate and a frame substrate. Japanese Patent No. 3011144 discloses an optical scanner in which fixed electrodes are positioned so as not to overlap with a mirror part in its directions of vibration. Such an optical scanner (a scanning mirror) is also disclosed in Herald Schenk; “An Electrostatically Excited 2D-Micro-Scanning-Mirror with an In-Plane Configuration of the Driving Electrodes,” the 13th Annual International Workshop on MEMS2000, 473-478 (2000). A torsional actuator that reduces driving voltage without changing the deflection angle of a mirror by providing an opposing electrode so that the opposing electrode is inclined from the center position of the deflection of the mirror is disclosed in the 13th Annual International Workshop on MEMS2000, 645-650 (2000), and Herald Schenk et al.; “A New Driving Principle for Micromechanical Torsional Actuators,” the 1999 ASME International Mechanical Engineering Congress and Exposition, 333-338, Nov. 14-19, 2000.
Japanese Laid-Open Patent Application No. 2003-15064 discloses a deflector mirror in which each torsion beam (torsion connection part) connecting a mirror substrate (a mirror formation part) and a frame is relatively wide at the connection to the mirror formation part and is gradually narrowed in a direction away from the mirror formation part at least up to halfway toward the frame, thereby preventing the rotation of the mirror substrate around a normal.
Published Japanese Translation of PCT International Application No. 2003-503754 discloses a deflector mirror (a micromechanical oscillating device) including a converter structure for converting external impact forces applied to a mirror substrate (an oscillating structure) into forces in the axial directions of a torsion beam (a torsion spring element) between the mirror substrate and the torsion beam.
Further, Japanese Laid-Open Patent
Application No. 2002-48998 discloses an optical scanning device that causes a light beam to be emitted after causing the light beam to be reflected multiple times between the mirror surface of a deflector mirror and a mirror surface opposite thereto.
Conventionally, in a mechanical element requiring a high-speed operation, its inertia is a great hindrance to drive speed. In particular, in a mechanical element that rotationally vibrates within a predetermined angle, it is necessary to reduce the moment of inertia. At this point, attention should be paid so as not to reduce the rigidity of the mechanical element to be driven. For this purpose, the mechanical element is provided with a hollow structure, or a reinforcing member is fixed to the mechanical element.
Laser printers using an optical deflector to perform optical scanning have become more sophisticated and reduced in size, so that it is required that the optical deflector be also reduced in size. As an optical deflector satisfying such a requirement, an optical deflector that deflects light by causing a micromirror employing micromachining technology to perform torsional vibration is proposed.
A micromirror employed in this type of optical deflector is required to be drivable at high speed and have high rigidity. If the rigidity of the micromirror is insufficient, the micromirror deflects greatly because of an inertia force generated with the vibration of the mirror. Such dynamic deflection extremely degrades the optical characteristics of the reflected light of the mirror. In general, this type of dynamic deflection is reduced by increasing rigidity by increasing the thickness of the mirror.
However, the acting force of an actuator employed in this type of optical deflector is extremely small. If the thickness of the mirror is increased to reduce dynamic deflection, the inertia of the mirror increases so that the small acting force of the actuator cannot prevent the angle of deflection from decreasing significantly. Accordingly, it is necessary to reduce the moment of inertia of the mirror in order to increase the angle of deflection.
Japanese Laid-Open Patent Application No. 2003-131161 discloses an optical deflector in which the moment of inertia is reduced by setting a vibration plate (a mirror substrate) so that its thickness gradually decreases outward.
Even in the above-described conventional technology, however, the moment of inertia is still great so that dynamic deflection cannot be controlled sufficiently.
FIG. 1 is a schematic diagram of a conventional deflector mirror. The deflector mirror of FIG. 1 includes a substrate 1, a rotary shaft 2 formed of torsion beams, and a rectangular mirror substrate 3 provided on the substrate 1. A thin metal film having sufficient reflectance with respect to employed light is formed on the mirror substrate 3 as a mirror surface so as to deflect light made incident on the mirror surface.
The mirror substrate 3 vibrates rotationally in a sine wave manner on the rotary shaft 2. At the time of vibration, an inertia force works because of the vibration of the mirror substrate 3. That is, dynamic deflection deformation occurs in the mirror substrate 3 because of an inertia force when the mirror substrate 3 vibrates in a reciprocating manner. This inertia force causes a bending moment Mx to act on each point of the mirror substrate as illustrated in FIG. 2. FIG. 2 is a schematic diagram of the bending moment acting on each point of the mirror substrate 3. FIG. 3 is a graph illustrating the results of calculating the bending moment Mx.
In FIG. 3, letting the length of the mirror substrate 3 in a Y-axial direction be L, the peak positions of the bending moment Mx are located approximately at ±L/3 positions.
A description is given, with reference to FIGS. 4A and 4B, of the dynamic deflection deformation of the mirror substrate 3, which is uniform in thickness and has a length of 2L with the center of each of opposing sides thereof being directly connected to the unsupported end of the corresponding torsion beam, in the case of causing the mirror substrate 3 to vibrate in a reciprocating manner around the rotary shaft 2.
FIG. 4A is a schematic diagram of the dynamic deflection deformation of the mirror substrate 3. FIG. 4B is a graph illustrating variations in the calculated value (solid line) and the actually measured value (dots) of the dynamic deflection of the mirror substrate 3 in its longitudinal directions. The deformation of the mirror substrate 3 was measured by actually causing the mirror substrate 3 to vibrate.
As illustrated in FIG. 4A, the mirror substrate 3 vibrates approximately in a sine wave manner so as to deflect like a wave. The calculated value of the dynamic deformation is the difference between the curved surface of the mirror substrate 3 and an ideal plane obtained from the curved surface of the deformed mirror substrate 3 by the least squares method. The calculated value was obtained by numerical calculation considering the inertia force of the mirror substrate 3. The actually measured value of the dynamic deflection is a value measured by holography. Here, the absolute values of the calculated and actually measured values are not illustrated because it is intended to illustrate how the dynamic deflection of the mirror substrate 3 varies in its longitudinal directions.
As illustrated in FIG. 4B, a peak of the dynamic deflection appears around a position approximately L/3 apart from the rotation center axis of the mirror substrate 3, which coincides with the extension of the center axis of the torsion beams. That is, a maximum deflection can be obtained around a position where approximately Y=L/3. Specifically, for instance, in the case of causing a mirror substrate of single crystal silicon 4 mm×4 mm in size and 20 μm in thickness, supported directly by two torsion beams, to vibrate in a reciprocating manner at a deflection angle of ±5° at 2.5 kHz, deflection in the proximity of a position L/3 distant from the rotation center axis amounts to no less than approximately 1 μm.
Such dynamic deflection deformation of a mirror substrate degrades the optical characteristics of a light beam reflected from a mirror surface formed as a thin metal film on the mirror substrate. Accordingly, it is necessary to prevent such dynamic deflection deformation as much as possible.
In order to reduce the dynamic deflection deformation of a mirror substrate, the rigidity of the mirror substrate may be increased by increasing its thickness. However, such an approach causes a problem in that the deflection angle (angle of deflection) of the mirror substrate decreases because of an increase in the moment of inertia of the mirror substrate or driving energy necessary for obtaining the same deflection angle increases (a driving voltage becomes high in a deflector mirror of an electrostatic driving type).